The present invention relates generally to separating centrifuges, and more specifically, to an improved screen basket for use in separating centrifuges.
Centrifuges, such as screen scroll centrifuges, are often used to filter or dewater crystalline or amorphous solid/liquid slurries. These centrifuges typically utilize a screen to separate the solid portion of the slurry from the liquid phase. The screen, moreover, is typically sized to retain the larger solids portion of the slurry while allowing the liquid to pass through and thus the two phases of the slurry may be separately collected. Instead of relying on Earth""s gravity to filter the slurry through the screen, however, filtration occurs under large centrifugal forces (on the order of many times the force of gravity), caused by high rotational speed of the centrifuge. These large centrifugal forces substantially increase the separation efficiency of the centrifuge.
Specifically, the slurry is delivered to the interior of a rotating basket that includes a conical screen portion. The conical screen portion is typically formed from a plurality of wire segments that are spaced side-by-side. For structural support, the wire segments may be welded to circumferential ribs spaced out along the axis of the basket. Rotation of the cone-shaped basket drives the slurry against the inner surface of the basket and the liquid phase is forced through the slots formed between adjacent wire segments. The larger solid particles do not pass through the slots and are instead collected on the inside of the basket.
To convey the solids out of the inside of the basket, a scroll conveyor having a helical blade is typically mounted concentrically within the basket. The tip of the blade, moreover, is spaced from the inner surface of the basket by a small radial clearance. The scroll conveyor is rotated in the same direction as the basket but at a slightly different rotational speed relative to the basket. Through this differential speed, the solids accumulating along the inside surface of the basket are conveyed by the helical blade from the small diameter end toward the basket""s larger end where they are dumped in a discharge chute and collected.
Another type of separating centrifuge is a vibrating centrifuge. Vibrating centrifuges also include a screen basket that is similar in design to the basket of screen scroll centrifuges. A vibrating centrifuge, however, does not utilize a helically bladed scroll to move the solid particles collecting on the inside surface of the basket to the discharge chute. Instead, the vibrating centrifuge includes a mechanism for shaking the basket back-and-forth along its axis. By shaking or vibrating the basket along its axis, solid particles accumulating on the inside of the basket are conveyed axially toward the discharge chute and collected.
As shown, scroll and vibrating centrifuge are very useful for separating liquid/solid slurries. Nonetheless, these centrifuges are subject to significant wear requiring frequent maintenance and corresponding down time. For example, solid particles of the slurry often get trapped in the slots of the basket, damaging the screen and reducing the separation efficiency of the centrifuge. Furthermore, the slurries often include highly abrasive components that wear out the screen portion of the basket. The corresponding maintenance and replacement of parts significantly increase operating costs.
An improvement to the conventional screen basket is described in U.S. Pat. No. 5,378,364 commonly owned by the assignee of the present application. In the ""364 patent, the conventional wedge wire basket is replaced with a wear resistant basket. In particular, the basket includes an outer structure having a plurality of closely spaced apertures or windows. Bridging each aperture or window are a plurality of wear resistant ligaments arranged to form screen sections. The ligaments may be formed from tungsten carbide which has improved wear characteristics as compared to the more conventional wedge wire. The tungsten carbide ligaments are typically spaced side-by-side inside the basket. The spaces between adjacent ligaments form slots through which the liquid portion of a slurry passes. The configuration and arrangement of the ligaments, moreover, reduce the chances of entrapment of solid particles in the slots.
Although the wear resistant basket of the ""364 patent presents a significant improvement in scroll centrifuges, it is nevertheless costly to manufacture. Specifically, as set forth above, the screen elements of the wear resistant basket are typically formed from tungsten carbide ligaments and extend the full length of the basket. Since tungsten carbide is a relatively expensive material, the cost of manufacturing such baskets is high. In addition, the ligaments are typically installed within the basket by hand further increasing the manufacturing costs of such baskets. Thus, it is desirable to develop a screen basket having wear resistant features but at a lower cost.
It is an object of the present invention to provide a screen basket for use in a centrifuge having improved wear-resistant characteristics.
It is a further object of the present invention to provide a centrifuge that is less costly to manufacture and operate than prior designs, while still having high wear resistant characteristics.
Briefly, the invention relates to an improved screen basket rotatably mounted within a centrifuge. The basket is preferably formed in the shape of a truncated cone and includes a small diameter segment and a large diameter segment. The basket also has two screen sections. In particular, the basket includes a first screen section formed from a wear resistant material. The first section is located at the small diameter segment of the basket proximate to the introduction point of slurry to the basket. A second screen section is located at the large diameter segment of the basket and is preferably formed from less expensive materials, such as steel or stainless steel wedge or profiled wires. The inner surfaces of the two sections are smoothly joined together so as to form a single continuous screen basket.
In a preferred embodiment, the first screen section preferably takes up approximately one-third of the length of the basket and the second screen section comprises the remaining two-thirds. Furthermore, the slots of the first section are preferably oriented along a circumferential direction, whereas the slots of the second section are oriented axially relative to the basket. The first section of the basket also smoothes the feed slurry as it impinges on to the basket surface. That is, the first section is configured and arranged to convert the discrete streams of slurry entering the basket into a smooth, continuous annular sheet, thereby facilitating the separation process. A solid wall section within the basket may also be employed at the feed introduction point to improve smoothening of the slurry and reduce wear.
A feed accelerating system may also disposed inside the basket to improve the operating efficiency of the centrifuge. The accelerating system preferably includes a rotating cone accelerator which receives slurry from a feed pipe. The inside surface of the cone accelerator includes a plurality of vanes for accelerating the slurry in a circumferential direction from the small end of the cone, where the feed is introduced, to the large end of the cone where the feed exits. The slurry exits the accelerating cone through passages located at the large diameter end of the cone and is preferably delivered to the basket.
To facilitate the discharge of slurry from the accelerating system, the vanes preferably include a terminal segment mounted along the trailing edge (relative to the direction of rotation) of the passages and extending radially inward. This terminal segment directs the slurry through the respective passage. An overspeeding member, extending outwardly in a generally radial direction from the trailing edge of each passage, may also be provided so as to accelerate the slurry exiting the accelerator system to the tangential velocity of the corresponding introduction point of the basket which is at a larger radius.
The small diameter segment of the basket, which is lined with wear resistant material, assists in smoothening the individual streams or jets of feed exiting the passageways in the cone in order to form a uniformly distributed (e.g., 360 degrees) sheet of slurry on the basket. During this smoothening process, the basket may be subject to substantial wear, especially with abrasive slurries, such as coal. Such potential wear, however, is resisted by the wear resistant material located at the small diameter segment of the basket.